Browsing by Author "Labuschagne, Pieter Willem Jacobus"

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    Impact of exogenous thiamine on Kluyveromyces marxianus under oenological conditions
    (Stellenbosch : Stellenbosch University, 2020-12) Labuschagne, Pieter Willem Jacobus; Divol, Benoit; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Managed inoculation of non-Saccharomyces yeast species is regarded as a practical way to introduce new characteristics to wine. However, these yeasts struggle to survive until fermentation is complete. The non- Saccharomyces strain, Kluyveromyces marxianus IWBT Y885, was recently isolated from South African grape juice and was partially characterised for winemaking purposes. This yeast strain displays relatively good fermentation ability as well as commercially relevant characteristics such as the production of strong pectinase activity and high phenylethanol and phenylethyl acetate production levels. In order to consider this strain for industrial winemaking, it is important to ensure its prolonged survival and good fermentation performance. A key factor for survival, growth and sustained metabolic activity of all yeasts is their nutrient requirements. Thus, identifying nutrients that are essential for maximising fermentation performance, and subsequently ensuring adequate levels of nutrients, is a means to ensure significant contribution of yeast to wine properties. This study aimed to identify essential nutrients, other than previously studied sugars and nitrogen, for maximum impact of K. marxianus Y885, as well as to characterise the outcomes of their utilisation. A multifactorial experimental design was employed to investigate the impact of nutrient concentrations on fermentation performance with K. marxianus Y885 in synthetic must. Exogenous thiamine concentration was determined to be the most significant factor impacting fermentation performance of K. marxianus Y885. The response to exogenous thiamine concentration for K. marxianus Y885 and S. cerevisiae EC1118 was compared in terms of population viability, fermentation rate, total sugars utilised, thiamine assimilation kinetics, and final wine composition. A saturation effect for initial thiamine concentration of K. marxianus Y885 fermentations was characterised, with a maximum fermentation rate and over 90% of available sugars utilisation obtained at 0.25 mg/L. A delayed uptake of thiamine at higher concentrations for K. marxianus Y885 suggested differential regulation of thiamine uptake compared to S. cerevisiae EC1118. In addition, different trends in metabolites produced between species suggest that thiamine concentration impacts the carbon metabolic flux differently in these two yeasts. To assess how this knowledge would translate to industrial winemaking conditions, the effect of maximum legal thiamine concentration to K. marxianus Y885/S. cerevisiae EC1118 sequential fermentations at semi-industrial scale in real grape must was investigated. No effect of thiamine supplementation on fermentation performance was observed, most likely as a result of the high native must thiamine content and lack of indigenous organisms that may compete for thiamine. However, thiamine concentration clearly differentially affected major volatile production of K. marxianus Y885 and S. cerevisiae EC1118. Notably, an increased fusel alcohol production of K. marxianus Y885 resulted from thiamine addition, which positively affects wine aroma. This study provides relevant information that may be used to ensure optimal utilisation of K. marxianus Y885 in oenology and aids in our understanding of how thiamine alters yeast metabolism.